Pulsed neutron logging tools are routinely used in oil- and gas-well-logging operations to test the physical characteristics of subsurface formations penetrated by a wellbore. These tools generally include a neutron generator that produces bursts of high-energy neutrons, and one or more radiation detectors at selected distances from the neutron generator for measuring the secondary radiation (e.g., inelastic gamma rays, capture gamma rays, epithermal neutrons, or thermal neutrons) resulting from interactions of the high-energy neutrons with the materials in and around the borehole.
The high-energy neutrons can be generated in fusion reactions of the hydrogen isotopes deuterium and/or tritium. To cause fusion reactions, in turn, ionized deuterium and/or tritium gas may be accelerated by an ultra-high-voltage electrical field towards a deuterium- and/or tritium-containing target. The ions can be generated from neutral deuterium and/or tritium gas in various ways; most conventional pulsed neutron generators employ impact ionization by high-energy electrons. In a traditional Penning ion source, for instance, a high-voltage pulse (e.g., having an amplitude of a few kilovolts) is applied between a cathode and anode to create an arc discharge that causes electrons to be emitted from the cathode and accelerated towards the anode, colliding with and thereby ionizing gas molecules along the way. Alternative approaches to creating the requisite free electrons include thermal emission from heated cathodes in so-called “hot-cathode” ion sources, or field emission, i.e., tunneling of electrons through a potential barrier lowered by a very strong electrical field.
The indirect process of first creating electrons and then creating ions through electron collisions with neutral gas molecules generally results in a somewhat variable time delay of a few microseconds between the application of an electron-generating voltage pulse and the resulting neutron burst, rendering precise control over the timing of the neutron pulses difficult. Other characteristics of the neutron pulses, such as their shape and total neutron output, may likewise be less predictable or controllable than is desirable for the intended logging operations. Additional drawbacks of various conventional pulsed neutron generators include limited neutron output (which may be due, e.g., in Penning ion sources, to the fact that the majority of the ions are di-atomic); high ionization voltages, which may subject the neutron generator to electrical stresses that decrease its reliability and/or lifetime; and bulk and complexity resulting from additional ion-source components (such as, in Penning ion sources, a magnet used to lengthen the electron paths to thereby increase collision efficiency).